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<head>
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    <title>Document</title>
    <style>
        div {
            width: 0;
            height: 0;
            border-top: 10px solid transparent;
            /* border-right: 10px solid transparent; */
            border-bottom: 10px solid transparent;
            border-left: 10px solid yellow;

        }
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</head>

<body>
    <div>

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</body>

</html>